This invention relates to transformer coupled circuits, in particular to circuits for reducing the distortion introduced by the non-linearity of a magnetic core transformer employed in coupling a drive circuit to a driven circuit.
In coupling one circuit, a drive circuit, to another circuit, a driven circuit, it is often desirable to employ a magnetic core coupling transformer. Such coupling transformers are used, for example, to isolate the drive circuit from the driven circuit, particularly to avoid the common mode problems that would introduce unwanted noise, and in order to match the impedance of the drive circuit to the impedance of the driven circuit. A significant problem with the use of coupling transformers in audio frequency circuits is that the magnetic core of an audio transformer introduces some non-linearity due to the inherent magnetic characteristics of the core material, which is typically iron or an iron-nickel alloy. As a practical matter this cannot be avoided by eliminating the magnetic core because the magnetic core is needed to provide adequate coupling in a transformer of practical size. While transformer design technology allows audio transformers to be built that will introduce low levels of distortion at low amplitude levels, they are not adequate for many high-quality audio circuit applications where very low distortion of higher amplitude signals is needed, for example, test and measurement instrumentation.
Aside from improving the quality of coupling transformers, the principal approach to reducing the aforedescribed difficulty has been to detect a portion of the distortion signal produced by the transformer and provide that as negative feedback to an amplifier driving a transformer. This has been done, for example, by feeding back a portion of the signal generated at the secondary winding of the transformer. However, this solution eliminates the isolation advantage of the coupling transformer. The feedback approach has also been implemented by providing the coupling transformer with an additional winding which produces a signal representative of the distortion, and feeding that signal back to an amplifier driving the transformer primary. To avoid both DC overload and drive amplifier instability in such a circuit, a low-frequency negative feedback path separate from the distortion signal negative feedback path has been provided. Examples of the foregoing can be found in Paullus U.S. Pat. No. 4,453,131, issued June 5, 1984.
However, a problem with any feedback approach is that its effectiveness is limited by the actual gain of the drive amplifier. Since a real amplifier cannot have infinite gain, a feedback approach cannot totally eliminate the transformer distortion. The use of an additional feedback winding on the transformer also has the problems that it introduces inherent phase delay which limits the effectiveness of the transformer at high frequencies, and of course requires a special transformer.
It is known that, in concept, the distortion introduced in a coupling transformer can be explained as a voltage drop produced by the flow of distortion current generated by the magnetic core of the transformer through the resistance of the primary winding of the coupling transformer and the source impedance of the circuit driving the transformer. Thus, if the winding resistance did not exist and the source impedance could be ignored, there would be no distortion produced. It is also generally known that active circuits can be made to exhibit negative impedance. It would be desirable if such a circuit could be utilized to, in effect, cancel the resistance of the primary winding of a coupling transformer and thereby eliminate transformer-produced distortion.